Here is an update on the MIT design for 10 cm spherical microbots for exploring the planets 1000 microbots would have the same launch volume as the Mars Spirit rover.
They could be used to explore caves and large areas. They have spinoff capabilities for helping the military to search caves and buildings.
An artists rendering of a microbot.
A swarm of microbots on Mars
Here is the project page at MIT
Hopping power is provided by a compliant bistable foot operated by Dielectric Elastomer Actuators
concept using 4 Dielectric Elastomer Actuators to change microbot orientation before hopping.